The Mockingbird Mine, Mariposa County, California

Collector’s Edge Minerals, Inc. recognizes and thanks Rocks & Minerals for permission to use portions or all of their original article from “Rocks & Minerals”, Volume 83, Number 5, pp. 392-400.  Contributing authors were Robert B. Cook, Department of Geology, Auburn University, Auburn, Alabama and Thomas M. Gressman, Collector’s Edge Minerals, Inc., Golden, Colorado. Collector’s Edge has made some additions and changes to the original article which are incorporated below.

Introduction

Gold has had a powerful influence on human beings throughout history.  Perhaps there is something in the neuronal wiring of our species that is the cause of this powerful attraction.  The color, luster and chemical stability of gold have never gone out of fashion over thousands of years, in numerous cultures spread around the planet.

For the avid mineral enthusiast, gold, especially crystallized gold, is one of the most coveted of all collectible mineral species.  However, of the thousands of known gold occurrences, few have produced aesthetic macroscopic crystals.  The Mother Lode area in California has produced many of the finest gold specimens known, including incredible examples from the Colorado Quartz, Red Ledge, Angel’s Camp, Sixteen-to-One, Eagle’s Nest, Diltz, Artru and Mockingbird mines.  These specimens commonly have bright luster and rich color, with well-developed crystals in aesthetic clusters.  Since Mother Lode gold usually occurs within milky quartz seams, fine matrix specimens show beautiful contrast between the gold and white quartz.

In today’s world of mineral shows and web sites it is not uncommon to see fine crystallized gold specimens offered or on display.  This is a relatively recent phenomenon; at major mineral shows in the 1970s essentially none were for sale.  We are living in the golden age of mineral collecting, and that is especially true with regard to gold crystals.  Mines like the Mockingbird had made the dreams of collectors who love crystallized gold specimens come true.

Collector’s Edge Minerals Inc., and Bryan Lees, have had a long relationship with John and Lucretia Emmett beginning with their work together at the Colorado Quartz mine. The Emmett’s have a standing agreement with Bryan and Collector’s Edge Minerals for preparation and marketing of selected gold specimens found at the Mockingbird mine. Many of the gold specimens discovered by John and Lucretia in 2006 and 2007 were prepared and marketed by Collector’s Edge at the 2007 Tucson Gem and Mineral Show. Bryan displayed five of the top specimens discovered by John in 2006. These specimens sold almost immediately, and two were actually sold before ever being displayed at the TGMS.

CEMI case display at 2007 TGMS with three fabulous gold specimens from John Emmett’s 2006 discovery.  Specimens were prepared and marketed by Collector’s Edge; case originally contained five specimens, but two were actually sold and removed from the case prior to the show opening.

Gold specimen sold to private collector prior to 2007 TGMS, exhibited by Collector’s Edge Minerals, Inc.

Location

The Mockingbird Mine is located in west-central Mariposa County, California, about 18 km almost due north of the town of Mariposa. It is west of California Highway 140 northwest of the community of Midpines. The mine, which is separated from the well known Colorado Quartz mine by the partial Artru patent, is in Section 27, T4S, R18E south of Saxon Creek. It is near the old mining camp of Colorado (Castello, 1921) and may be assigned to either the Colorado or the Whitlock district (Clark, 1970). The Mockingbird and surrounding mines are privately owned and are not open to collecting.

Map of the Mockingbird mine surface and partial underground workings.  Inset:  General location map of the mine.  Prepared by William Besse.

History

Various regions in California were mined by Sonoran Mexicans for gold prior to 1840.  Following the California Gold rush of 1849, Mariposa became a gold rush boom town.  Miners during this era followed alluvial gold back to their respective mother lodes, but not always resulting in successful discoveries.   Today old workings from these early gold miners can be found around the town of Mariposa.  The most prolific claims followed the Colorado dike producing pockets of exquisite crystalline gold specimens.  Of those remarkable claims the Mockingbird shares the distinction with the Colorado Quartz for producing some of the finest crystallized gold specimens ever found.  Unfortunately there is little published information in the scientific and mineral collecting literature.

The Mockingbird mine, also known as the Mocking Bird Quartz, was called the Talc and Lacey claim prior to 1900 (Bancroft 1987).   The patented Mockingbird claim is adjacent to the Colorado Quartz and Artru mines.  All three of these mines exploit the Colorado Dike and all have produced splendid crystallized gold specimens.   The headframe of the Colorado Quartz mine can easily be seen from the road leading to the Mockingbird property.  Unfortunately, most of the fine specimens that came out of the Mockingbird mine in the early 1890s and around the turn of the century have been melted down (Bancroft 1987).  The Mockingbird mine was then dormant for a period of time, and consequently was not known for producing “world class” gold specimens.  However, during one brief period of operation, the Weston Brothers produced a well-publicized pocket containing about 145 ounces of crystallized gold from the mine in 1915.

In the mid-1980s, in part due to success at the nearby Colorado Quartz mine, the Mockingbird Gold Mining Company was formed by Philip A. Rivera to operate the Mockingbird and adjacent Artru mines.  A crew from the silver district of Kellogg, Idaho, was imported, and modern machinery was put in place.  However, little was produced, and the property lapsed into dormancy.  In general, then, toward the close of the twentieth century the Mockingbird was still not known for producing “world-class” specimens.

Everything changed when John and Lucretia Emmett acquired the Mockingbird claim in 1996.  John, a road-building contractor in Clovis, California has a passion for hunting gold specimens. He is also one of few mine owners who understands the economic and historical value of high-quality specimens.  John is not a novice in this endeavor considering that in 1994 he became part owner of the Colorado Quartz mine and began surface mining there, resulting in the discovery of enough crystallized gold to get the mine back on its feet.  John was also instrumental in the discovery of the famous Colorado Quartz mine specimen called “the Dragon,” sold by Collector’s Edge Minerals, Inc. and now in the Houston Museum of Natural Science.  With the experience gained at the Colorado Quartz mine John developed a carefully designed strategy for finding crystallized golds. Using metal detectors and following many other subtle signs underground, John hit the Mockingbird “Mother Lode” in 2006.

An extraordinary group of approximately 26 crystalline gold specimens (thumbnail to cabinet size) were recovered from clay-filled pockets.   The finest pieces were collected in April 2006 from the third level, 30 meters below the surface.

The workings of the Mockingbird mine reflect its history. The oldest workings consist of shallow open cuts and underground drifts with associated stopes, some of which remain intact.  Early exploration underground for pockets focused on the Colorado dike and resulted in the development of an almost chaotic array of small tunnels and stopes, some with original timber still in place. The depth of these early workings was quite shallow, generally no more than 30 or 40 meters below surface. Modern efforts to systematically work the mine employed experienced miners and modern equipment but produced little gold and drove only relatively insignificant new underground openings until about 1996, the beginning of the Emmett era.

Today, the mine exhibits considerable new surface and underground development. The outcrop of the Colorado dike has been systematically worked to a depth of up to 8 meters from the south end of the property to near the new decline. This work, with the aid of metal detectors, resulted in the discovery of numerous gold-bearing pockets that had been missed by earlier miners.

Within the wide trench-like excavation on the dike are now two shafts. One is a manway/ventilation shaft that connects with the first accessible level through which the pay zone that produced the magnificent 2006 specimens can be accessed. This zone today is a gaping stope about 25 meters vertically by 30 meters along the strike of the western dike-slate contact. One can wind his way through the myriad of slusher drifts at this level to the second shaft which is located at an intermediate point between the first shaft and the decline. The bottom of this shaft, which is lined with large-diameter corrugated culvert pipe, served as a waste-rock pocket during the development of the stope in which the 2006 and related specimens were found. Rock was hoisted from this pocket with a crane and clamshell bucket, eliminating the need for a headframe and hoist.

About 100 meters northwest from the second shaft along the strike of the dike one drops into a narrow ravine. In this ravine is an open cut, at the end of which is the entrance to the current decline, an initially downward-sloping adit that is being driven to intersect the down-pitch projection of the 2006 pay zone approximately 30 or 40 meters below the bottom of the existing stope. Although the decline meanders across the dike at several places, there are no significant drifts or crosscuts. Near its entrance, however, a winze connects with deeper underground workings that are of relatively limited extent; near the end of the decline is a raise that allows ventilation through a network of shallower workings and the ventilation shaft mentioned above. At the time of this writing, the pay zone had not been intersected by the decline/adit.

Entrance into the 4th-level decline.  This opening and associated workings have been driven beneath the 2006 productive stope to explore for the downward projection of the pocket zone.  Head miner Noble Sparks (left), Tom Gressman and Tim Sparks in foreground.  Robert Cook photo.

The Mockingbird mine lies in a subsidiary district immediately east of the southern part of California’s famous Mother Lode system (Ransome, 1900; Knopf, 1928). The Mother Lode proper extends down the west flank of the Sierra Nevadas for more than 150 kilometers, from near Georgetown, El Dorado County on the north to a few kilometers south of Mariposa in Mariposa County. Its persistent quartz lodes are generally related to the Melones Fault zone or system whose trace is marked commonly by bold outcrops of north-trending milky quartz veins.  In Mariposa County, the Mother Lode is bounded on the east and west by parallel flanking groups of interrelated gold-quartz veins that collectively form the “east” and “west” gold belts. The Mockingbird mine is near the western edge of the east gold belt and, like most of the other gold specimen-producing mines for which California is famous, such as the Eagles Nest, Red Ledge, Colorado Quartz, Diltz, 16-to-1, and Oriental, it does not lie in the Mother Lode itself.  Mariposa County geology has been described in varying detail by Bowen and Gray (1957), Lowell (1916), and Preston (1890), to who the reader is referred for additional details.

General view of the Colorado dike where it is crossed perpendicularly by workings on the 4th level.  Note the subparallel floor veins that are dipping toward Tom Gressman; Tim Sparks on the right.  Robert Cook photo.

Oblique view of floor veins in the Colorado dike where they are exposed in the 4th-level workings.  Here the veins are cut by later joints along which there has been local movement consistent with the general orientation of cutters or control structures.  Robert Cook photo.

Tom Gressman at the beginning of the large 2006 stope on the 3rd level.  This stope produced the exceptional crystallized gold specimens displayed at the 2007 Tucson Gem and Mineral Show.  Robert Cook photo.

Shaft supplying access and ventilation to the 2006 3rd-level stope.  The shaft is lined with corrugated pipe shown projecting above the surface; black plastic protects the ventilation fan.  The shaft is the southernmost shown on the mine sketch map and is in the bottom of the shallow surface cut along the Colorado dike. 

Two dominant, though strikingly different lithologies host and/or control the location of gold-bearing quartz veins at the Mockingbird mine. The productive and gold pocket-bearing parts of quartz veins are at or adjacent to a northwest-trending, intensely altered mafic dike, the Colorado dike, that extends apparently uninterrupted from the Colorado Quartz mine across the Artru property and on through the entire length of the Mockingbird claim.  The dike is hosted by a silty graphitic slate known locally as the Briceburg formation. The Briceburg formation appears to be equivalent to the Mariposa formation of Ransome (1900), and is likely of late Jurassic age; it contains minor interbedded sandstone and conglomerate.

The Colorado dike varies locally in thickness and attitude but in the immediate mine area dips steeply northeast or southwest and strikes from N50°W to N70°W. Compositionally, it has been described by Kampf and Keller (1982) as a greenstone, a mafic rock that has been “propylitically altered.” Both the brittle altered dike and the immediately adjacent slates have undergone mineralization during a weak post-Jurassic folding event. At the surface the dike is deeply weathered and is cut by narrower though similar barren dikes of apparently the same generation.  In the active decline and associated workings it can be seen to vary in thickness from about 1 meter to as much as 13 meters. Its contact with the slate host is locally quite sheared and marked by swarms of subparallel, generally barren quartz stringers, particularly on the northeast side; groups of subparallel, pocket-bearing quartz veins within the dike itself show apparent drag folding against the contact.

Southwest of the Colorado dike is a banded quartz vein known as the Colorado vein. This vein, which is not known for crystalline gold but has been the source of high-grade gold ore, can be traced locally on the Mockingbird property. It strikes to the northwest, like the Colorado dike, but projects to a convergence with the Colorado dike beneath the current workings.

The productive quartz veins at the Mockingbird mine are locally called “floor” veins, a carryover from similar veins in other pocket mines where miners commonly use the shallow dipping veins as the floor for stopes.  At the Mockingbird mine, these veins vary from about 2 cm to as much as about 25 cm thick and are almost everywhere unmineralized and locally vuggy, and consist of milky white quartz. They are generally restricted to the dike proper although they locally cross the dike-slate contact, penetrating the slate for several meters. When viewed in a vertical plane perpendicular to the strike of the dike (looking into the dike in an up-strike or down-strike direction) these quartz veins, appear to lie more or less horizontally across the dike, making a right angle with the dike walls except where minor drag folds have deflected them downwards. When viewed from the side (looking at the dike-slate contact), they are seen to dip gently to moderately (up to 50 degrees) to the northwest. The most productive parts of these veins have been at the dike-slate contacts, generally on the western side of the dike.  The intense search for gold-bearing pockets in mines along the Colorado dike has resulted in the identification of poorly understood control structures called “cutters” by the miners. These are near-horizontal to nearly vertical, variably quartz-filled fractures that strike across the dike at generally shallow angles, intersecting dike-slate contacts at gentle angles. The most productive areas are those where “cutters” intersect “floor” veins at or very near the dike-slate contact.

At least 80 gold-bearing pockets and many more vuggy but barren pockets in floor veins have been found since the period of modern mining began under the present ownership in the mid-1990’s. Most productive pockets have been small but generally characterized by crystalline gold, the most dramatic discovery being a series of pockets found in April 2006.  Pockets are generally irregular in shape and small (up to about 20 cm) although barren vugs up to about a meter in maximum dimension have been found. They are commonly lined with quartz crystals, and those that contain gold are usually filled with a mixture of iron and manganese oxides and clay that can vary from red, through greenish and gray to almost black.

The spectacular discovery in 2006 was in a “pay” zone about 6 x 12 meters on the third level of the mine, some 30 meters below the surface and exterior to the southwest side of the dike. A string of pockets was found here in a fold in the dike. A seam of clay 15-25 cm wide filled the dike fold. Small grains of gold were first noticed in the clay, indicating that perhaps a pocket of gold crystals was nearby. Two of the finest specimens were found about 4.5 to 6 meters above the fold. All other pieces were found closer to the fold itself and range in size from a couple of millimeters to cabinet-size specimens. Crystallized masses of gold up to nearly a kilogram were found associated with milky quartz and arsenopyrite. Most pieces were found in vugs completely covered with fine-grained earthy greenish chlorite. The best pieces were found in a vug filled with black clay of unknown mineralogy; the major vugs were arranged as if at the corners of a three-dimensional cube tilted about 20 degrees from the horizontal. Metal detectors were instrumental in locating specimens within a meter of the pockets themselves in all four corners of this imaginary cube.

The gold occurs in a variety of habits that range from poorly crystallized, hackly masses through well-crystallized arborescent groups with brilliantly lustrous, flattened, and skeletal octahedrons somewhat reminiscent of Eagles Nest mine gold, to spectacular chunky masses adorned with dark yellow, mirror-bright crystals of unusual form, perfection, and size.  The gold occurs either free of matrix or intergrown with white massive, or rarely transparent, colorless, well-crystallized quartz.  As one might expect, its analogous geology and near proximity to the Colorado Quart mine suggest that Mockingbird mine gold will be quite similar to that from the former mine as described by Kampf and Keller (1982), and it is.  Crystals to almost 2 cm across have been found.  Octahedra are commonly skeletal and exhibit raised ridges marked by repetitive parallel hexoctahedral faces.

Gold:  Au

In 2007 an additional number of “world class” gold specimens were discovered at the Mockingbird.  The best specimen of this find, named the “Golden Coral,” is distinctly different in that it occurred as a floater in a vug filled with black clay. This cabinet piece has extraordinary color with a very rare level of mirror-brilliant luster. The “Golden Coral” was unique and so spectacular that it was sold immediately and never displayed publicly. However, the specimen now resides in the MIM Museum located in Lebanon and is now on display for all to enjoy!.

A fine 8.6 cm-wide gold on quartz.

Gold crystals growing up from matrix – 11.7 cm-wide.

Unusual, but fine 0.8 cm skeletal gold crystal on quartz.

An aesthetic 5 cm-wide crystallized gold on quartz.

Mass of crystallized gold wrapped around a quartz matrix – 6.0 cm x 2.2 cm.

A 5.9 cm-tall crystallized mass of gold.

Aesthetic group of crystallized gold on quartz matrix – 4.6 cm-wide.

A 3.2 cm specimen of crystalline gold on quartz.

A 2.2 cm specimen of crystalline gold.

Other Minerals

The mineralogy of the Mockingbird mine veins is quite simple, consisting of quartz, arsenopyrite, galena, pyrite, pyrrhotite, calcite, the previously mentioned ill-defined clay and chlorite family minerals, iron and manganese oxides. Ankerite has been described from the nearby Colorado Quartz mine. Soluble blue and green copper sulfates are reported in shallow parts of the workings, apparently after primary chalcopyrite.

Arsenopyrite :  FeAsS

Arsenopyrite is commonly associated with pockets, particularly in areas where they are gold-bearing. In general, the arsenopyite is coarse-grained but massive.

Calcite:    CaCO₃

Calcite occurs in good crystals to 2 cm across with quartz crystals in floor vein vugs. The crystals are often colorless and transparent, although creamy tan to yellow crystals were seen at the mine in attractive specimens.

Galena:   PbS

Galena occurs as nondescript small masses in floor veins near pockets. It is rare and is unknown in collectible specimens.

Pyrite:   FeS₂

Pyrite occurs sporadically in host and dike rock near quartz veins and locally in fine-grained nodular masses in the Briceburg formation. What are thought to be pyritized fossil shells have been found by the miners.  Collectible pyrite specimens are unknown from the mine.

Pyrrhotite:   FeS_1-x

Pyrrhotite has been reported by the miners as massive, narrow stringers associated with the margins of some floor veins. It was first noted because of its interference with metal detectors, giving false positive signals in discrimination mode.

Quartz:   SiO₂

Massive milky quartz is the dominant vein mineral. Vugs are common in floor veins near the contact between the Colorado dike and the enclosing slates. According to Bancroft (1987), miners had recently recovered 360 kg of quartz crystals, presumably from a single pocket or pocket zone. Individual crystals are up to 4 cm across and groups are up to 25 cm. Many crystals contained chlorite inclusions and phantoms. Limpid, mirror-faced, chlorite-included quartz crystals seen by the authors are up to 6 cm in diameter and 9 cm long.

Current Status

The fabulous specimens found at the Mockingbird mine since 1996 clearly rank it as one of the world’s greatest localities for crystallized gold. John Emmett and his miners, Noble and Tim Sparks, continue to seek out more elusive treasures.  Given the experience and past successes, the probability is high that more incredible gold specimens from the Mockingbird will be forthcoming.

References